Benzyl derivatives hydrogenolytic cleavage

Better inductions by a vicinal amino acid were observed by Ojima and coworkers in the benzylation of chiral /3-lactam ester enolates (255, equation 67) °. Interestingly, the enolate formation occurred at an uncommonly high temperature (0°C) to form the thermodynamic Li-chelated enolate 256, which allowed a stereoselective attack of the electrophile, while the diastereoselectivity with the nonchelated kinetic enolate 259 was significantly lower. Subsequent hydrogenolytic cleavage of lactam 257 delivered S)-a-methylphenylalanine derivative 258 in nearly quantitative yield and high diastereoselectivity. 

Davies pioneered a versatile method to prepare chiral /S-amino ester derivatives through diastereoselective conjugate additions of chiral amines onto unsaturated esters [33, 101]. Conjugate addition of lithium amide 103 to acceptor 102 thus afforded an adduct in 82 % yield, and after hydrogenolytic cleavage of the N-benzyl group this provided -amino ester 104 in >98% ee [102]. Such asymmetric amide additions have been demonstrated to have wide substrate scope with respect to the substituents that may be employed on the a,y5-unsaturated esters [33]. 

The obtained C-lactosyl tyrosine derivative 29 was further transformed by hydrogenolytic removal of the benzyl groups, O-acetylation, acidolytic cleavage of the Boc and tert-butyl ester groups and selective introduction... 

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